Curing characteristics and internal stresses in epoxy coatings: Effect of crosslinking agent

The development of internal stresses in a series of epoxy coatings prepared using five different crosslinking agents have been studied. The crosslinking agents were: H1, 4,4 methylenedianiline (DDM); H2, diethylentriamine (DETA); H3, cycloaliphatic polyamine; H4, polyaminoimidazoline; and H5, polyamidoamine adduct. Four different post-cure treatments were applied and the dependence of internal stress on crosslinking agent and post-cure treatment was determined. Curing was followed by monitoring the FTIR epoxy band at 916 cm^{– 1} and the glass transition temperature was determined using DSC to assist interpretation of the measured values of internal stress. The internal stress was tensile in all of the materials at the end of each post-cure treatment. The stress magnitudes increased monotonically with post-cure temperature. The largest stresses were recorded with H1, H2 and H3 whereas the lowest stresses were recorded with H4 and H5, which both included a flexible aliphatic chain. The effects of ageing for extended periods in dry air and in humid air (52%RH and 97%RH) were also examined. Exposure to humid air almost always caused a reduction in the tensile stress and often produced compressive stresses, attributed to swelling due to water absorption. A comparison was made of the stresses formed in coatings applied to a thin substrate that was (i) free to bend during curing, post-curing and ageing, and (ii) prevented from bending (restrained substrate). The general trends in behaviour were in agreement but no simple relationship could be found between the stress magnitudes obtained by the two different test configurations.